If I understand the issue correctly, the proposed flange is similar to that shown in VIII-1 App 2 Fig 2-13 with the bolt hole and gasket surface reversed. I haven't pondered the implication much, but seems that the internal moment caused by the bolt-gasket couple is behaving the same way as it would for a normal flange. Thus, I'd start with a "normal" flange calc. The only thing substantially different is the flange is relatively smaller and, of course, needs to have internal access on both sides for bolt-up and will interfere with the fluid flow like an RO would.
jt